Metal pipes typified by steel pipes are produced by hot working such as piercing-rolling and hot extrusion, and as needed, cold working such as cold drawing is carried out. The metal pipes are further subjected to heat treatment typified by quenching etc., to obtain desired mechanical properties (strength and toughness, etc.).
One method for heat treating a metal pipe is quenching by induction heating. Induction heating is carried out by using an induction heating coil. The metal pipe is heated while being passed through the induction heating coil. The heated metal pipe is cooled by water cooling etc. to be quenched.
In quenching, it is preferable to heat the entire metal pipe as uniformly as possible. However, in the case of quenching by induction heating, the pipe end portions of the metal pipe in particular are not likely to be uniformly heated. In the case of induction heating, unlike a batch processing by a batch furnace, the metal pipe is heated while it passes through in an induction heating coil. Therefore, the pipe end portions of the metal pipe are not likely to be heated equally with the portions of the metal pipe other than the pipe end portions, and may result in underheating or overheating. In such a case, an objective micro-structure cannot be obtained in the pipe end portions. Moreover, if a coolant such as water and oil enters onto the internal surface of the pipe from a pipe end during cooling after induction heating, the objective micro-structure may not be obtained in the pipe end portions.
In the quenching method by induction heating disclosed in JP2006-233303A and JP60-29421A, a plug or jig (hereafter, referred to as a plug etc.) is disposed between steel pipes arranged fore and aft in a line so that fore and aft steel pipes are connected with the plug etc. Thus, continuous heat treatment is carried out by induction heating on the mutually connected steel pipes. As a result of this, firing unevenness of the pipe end portions is suppressed, and further, the entry of the coolant onto the internal surface of the pipe is inhibited by the plug etc.
In the quenching method disclosed in JP62-246282A, a preceding steel material and a succeeding steel material are subjected to induction heating while they are conveyed with the spacing between them being kept at a predetermined distance. In this case, the front end portion of the succeeding steel material is preheated by the radiant heat from the rear end portion of the preceding steel material. As a result, underheating of the pipe end portions is suppressed, and the length of unquenched portion in a pipe end portion is reduced.
However, in the quenching method disclosed in JP2006-233303A and JP60-29421A, the plug etc. needs to be connected to pipe end portions. Therefore, it is difficult to increase productivity. Moreover, when the metal pipe is bent globally or partly, or the pipe end face of the pipe has unevenness, it is difficult to connect a plug etc. to a pipe end portion. Since, as described above, the metal pipe is produced by hot working, cold working, and the like, the metal pipe may be bent globally or partly during the production process. Moreover, the pipe end face after production may not be flat, and may have unevenness. In such a case, quenching defects such as firing unevenness are likely to occur in a pipe end portion.
In the quenching method disclosed in JP62-246282A as well, if the metal pipe is bent globally or partly, or the pipe end face of the pipe has unevenness, the radiant heat of the rear end of the preceding steel material is not likely to transfer to the front end of the succeeding steel material. Therefore, quenching defects such as firing unevenness are likely to occur at a pipe end portion.